Re: [PATCH] timers/core: Correct callback order during CPU hot plug.

[Jon Hunter]

Hi Richard,

On 27/07/16 10:08, Richard Cochran wrote:
> On the tear-down path, the dead CPU callback for the timers was
> misplaced within the 'cpuhp_state' enumeration.  There is a hidden
> dependency between the timers and block multiqueue.  The timers
> callback must happen before the block multiqueue callback otherwise a
> RCU stall occurs.
> 
> This patch fixes the order within the hot plug state machine.
> 
> Signed-off-by: Richard Cochran <rcochran@xxxxxxxxxxxxx>
> Fixes: 24f73b99716a ("timers/core: Convert to hotplug state machine")
> 
> Cc: John Stultz <john.stultz@xxxxxxxxxx>
> Cc: Jon Hunter <jonathanh@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: rt@xxxxxxxxxxxxx
> ---
>  include/linux/cpuhotplug.h |  2 +-
>  kernel/cpu.c               | 15 ++++++++++-----
>  2 files changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
> index 6d405db..242bf53 100644
> --- a/include/linux/cpuhotplug.h
> +++ b/include/linux/cpuhotplug.h
> @@ -20,9 +20,9 @@ enum cpuhp_state {
>  	CPUHP_PROFILE_PREPARE,
>  	CPUHP_X2APIC_PREPARE,
>  	CPUHP_SMPCFD_PREPARE,
> -	CPUHP_TIMERS_DEAD,
>  	CPUHP_RCUTREE_PREP,
>  	CPUHP_NOTIFY_PREPARE,
> +	CPUHP_TIMERS_DEAD,
>  	CPUHP_BRINGUP_CPU,
>  	CPUHP_AP_IDLE_DEAD,
>  	CPUHP_AP_OFFLINE,
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 67f4943..ec12b72 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1208,11 +1208,6 @@ static struct cpuhp_step cpuhp_bp_states[] = {
>  		.startup = smpcfd_prepare_cpu,
>  		.teardown = smpcfd_dead_cpu,
>  	},
> -	[CPUHP_TIMERS_DEAD] = {
> -		.name = "timers dead",
> -		.startup = NULL,
> -		.teardown = timers_dead_cpu,
> -	},
>  	[CPUHP_RCUTREE_PREP] = {
>  		.name = "RCU-tree prepare",
>  		.startup = rcutree_prepare_cpu,
> @@ -1229,6 +1224,16 @@ static struct cpuhp_step cpuhp_bp_states[] = {
>  		.skip_onerr		= true,
>  		.cant_stop		= true,
>  	},
> +	/*
> +	 * On the tear-down path, timers_dead_cpu() must be invoked
> +	 * before blk_mq_queue_reinit_notify() from notify_dead(),
> +	 * otherwise a RCU stall occurs.
> +	 */
> +	[CPUHP_TIMERS_DEAD] = {
> +		.name = "timers dead",
> +		.startup = NULL,
> +		.teardown = timers_dead_cpu,
> +	},
>  	/* Kicks the plugged cpu into life */
>  	[CPUHP_BRINGUP_CPU] = {
>  		.name			= "cpu:bringup",
> 

This fixes the RCU stalls I was seeing during suspend on Tegra and so ...

Tested-by: Jon Hunter <jonathanh@xxxxxxxxxx>

Cheers!
Jon

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